The present invention relates to a phase mask, and particularly relates to a phase mask for use in a holographic apparatus for recording a high-density image information for a Fourier transform hologram.
There have hitherto been proposed a variety of practical holographic apparatuses which generally utilize various properties of hologram such as high storage density and redundancy of memory, self-focalizability of reconstructed image, etc. However, all of these conventional holographic apparatuses have been defective in that, because of the noise arising from a flaw or dust in the optical system attributable to the spatial and temporal coherence of the laser beam used, the coherent noise represented by the speckle noise arising from the diffusing surface, and the non-linear record noise arising from non-linear recording on a hologram recording material, the resulting image has a great defect in quality which has been a major obstacle to the practical use of the holographic apparatus.
Under such circumstances, various attempts have so far been made with a view to eliminating the noise and improving the quality of image. To cite an instance, it has been proposed to apply the method of superposing a random phase mask upon a two-dimensional transparent object at the time of photographing a Fourier transform hologram of a two-dimensional transparent object as the data mask.
By the way, there are known various types of random phase masks of this kind. All of these conventional random phase masks are fabricated through a process in which a mask with a random pattern is first prepared according to the random function generated by a computer, and next a transparent dielectric substance like zinc sulfide (ZnS), cerium oxide (CeO.sub.2), etc., is deposited in a few or several steps through vacuum evaporation on an optical glass plate through this mask. The preparation of the conventional random phase masks is very difficult technically and is apt to be costly. Besides, these phase masks have various drawbacks such that the thickness of the transparent dielectric substance for use in the phase mask must be changed according to the wavelength of the laser beam to be applied, and therefore, the resulting phase mask is insufficient in function for use in recording a color image. Moreover, it is insufficient in randomness, so that there occurs the localization of luminant beams from the object on the Fourier transform plane or the generation of Moire effect ascribable to the sampling mesh used jointly with the phase mask.